Config changes does not work over ACP

### Describe the bug

After starting `copilot --acp`, initializing ACP, and creating a new session with session/new, calling session/set_config_option fails for permission-related config.

Example request:

{
"jsonrpc": "2.0",
"id": 3,
"method": "session/set_config_option",
"params": {
"sessionId": "",
"configId": "allow_all",
"value": "on"
}
}

Response/error:

Failed to set config option 'allow_all' to 'on': Error: Permission service is unavailable for this session.

### Affected version

1.0.54

### Steps to reproduce the behavior

Script to reproduce it:

```
#!/usr/bin/env bash
set -euo pipefail

# Minimal Copilot ACP repro:
# 1. start copilot --acp
# 2. initialize
# 3. create a session
# 4. try to set allow_all=on
#
# Expected failure:
# Failed to set config option 'allow_all' to 'on': Error: Permission service is unavailable for this session.

node <<'NODE'
const { spawn } = require("node:child_process");

const child = spawn(process.env.COPILOT_ACP_COMMAND || "copilot --acp", {
shell: true,
stdio: ["pipe", "pipe", "pipe"],
});

let nextId = 1;
let buffer = "";
const pending = new Map();

child.stderr.on("data", (chunk) => process.stderr.write(chunk));
child.stdout.on("data", (chunk) => {
buffer += chunk.toString();
const lines = buffer.split(/\r?\n/);
buffer = lines.pop() || "";
for (const line of lines) {
if (line.trim() !== "") handleLine(line);
}
});

child.on("exit", (code, signal) => {
for (const { reject } of pending.values()) {
reject(new Error(`copilot exited: code=${code} signal=${signal}`));
}
});

function request(method, params) {
const id = nextId++;
child.stdin.write(`${JSON.stringify({ jsonrpc: "2.0", id, method, params })}\n`);

return new Promise((resolve, reject) => {
pending.set(id, { resolve, reject });
setTimeout(() => reject(new Error(`timeout waiting for ${method}`)), 60000);
});
}

function handleLine(line) {
const message = JSON.parse(line);

if (message.id !== undefined && message.method) {
// This repro does not implement client-side ACP methods.
child.stdin.write(`${JSON.stringify({
jsonrpc: "2.0",
id: message.id,
error: { code: -32601, message: `not implemented: ${message.method}` },
})}\n`);
return;
}

if (message.id === undefined) return;

const waiter = pending.get(message.id);
if (!waiter) return;
pending.delete(message.id);

if (message.error) {
waiter.reject(new Error(message.error.message || JSON.stringify(message.error)));
} else {
waiter.resolve(message.result);
}
}

(async function main() {
await request("initialize", {
protocolVersion: 1,
clientCapabilities: {
fs: { readTextFile: false, writeTextFile: false },
},
});

const session = await request("session/new", {
cwd: process.cwd(),
mcpServers: [],
});

console.log(`sessionId=${session.sessionId}`);
console.log("setting allow_all=on...");

try {
await request("session/set_config_option", {
sessionId: session.sessionId,
configId: "allow_all",
value: "on",
});
console.log("unexpected success");
process.exitCode = 1;
} catch (error) {
console.error(String(error.message || error));
process.exitCode = error.message.includes("Permission service is unavailable")
? 0
: 1;
} finally {
child.kill("SIGTERM");
}
})();
NODE
```

### Expected behavior

It should allow mode to be changed.

### Additional context

This is a separate but related issue with #2942. #2942 is about the model not being able to leave plan mode. This one applies to all configs in general. Thanks!!!

Start by running the provided Node.js reproduction against `copilot --acp`, then trace the `session/new` and `session/set_config_option` entry points and how the permission service is initialized for ACP sessions. Done means changing `allow_all` to `on` succeeds without the permission-service-unavailable error, while the existing configuration behavior remains intact.

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